A conventional tension mechanism for a packing machine generally includes a tension bar which is rotated with a transmission cam, a passive link, a spring, a pivot link, a clamping member, a driving member driven by a motor, a driving motor which is controlled by a controller and can be operated in forward and backward directions, an activation member on the transmission shaft for activating the controller, and a relay connected to the driving motor. When the driving motor is activated, the driving member drives the passive link to move a distance so as to push a clamping member to clamp the band which is packing an object and the tension bar is rotated so that the controller is activated by the activation member. The relay delays the activation of the driving motor so as to control the tension of the band.
However, in order to provide sufficient space for the rotation of the tension bar, the horizontal space of the machine has to be large enough and this limitation requires more space of the work site. When the guide mechanism pulls the band to pack the objects, the transmission shaft is rotated to control the critic position of the link of the tension bar relative to the transmission cam, a first controller sends a signal to the relay to activate the driving motor after a pre-determined delayed period to drive the driving member to rotate the link of the tension bar, so that the clamping member clamps the band. The sequence of the actions of the above mentioned parts and the time required to allow the tension bar to rotate results a long operation period which is needed to be shortened for efficient purposes. The teeth of the clamping member might damage the bands if they are too sharp, and cannot clamp the bands in position if the teeth are dull. The clamping member is pivotably connected to an end of the tension bar and the band is clamped between the clamping member and the inside of a chamber. The precision of the installation of the clamping member in horizontal direction and the surface of the chamber are required to be highly machined, or the bands cannot be pressed as desired. This required high-cost machining processes to manufacture the related parts.
The present invention intends to provide a forward-reverse tension mechanism which includes less number of parts, occupies less space and shortens the response time.